The present invention relates to fluid pumps and, more particularly, a variable displacement pump suitable for use in automobiles.
Power-assisted steering systems are systems used to aid drivers in controlling vehicles on the roads at low and high speeds by providing assistance to drivers in turning the steering wheel under various conditions. Power steering systems typically comprise a rack-and-pinion steering gear mechanism in which the gear rack is connected to a steering gear linkage and to the piston of a fluid motor. The rack engages a pinion gear that is connected to a driver operated steering shaft. A power steering pump is typically coupled to the rack-and-pinion gear mechanism to provide steering assistance to the rack-and-pinion gear mechanism as necessary. The power steering pump uses a rotary valve mechanism to control distribution of pressure from a power steering pump to the fluid motor portions of the rack-and-pinion steering gear mechanism. One type of power steering pump typically used in automotive systems is the constant displacement rotary vane pump.
Constant displacement rotary vane pumps of the type used in power steering devices have a flow rate proportional to rotor speed. The steering gear supplied with pressurized hydraulic fluid from the pump requires high flow rates when vehicle speed is low and low flow rates when vehicle speed is high. With a constant displacement pump, however, the flow of hydraulic fluid from the pump is controlled by the rotor speed, and not by the amount of steering assistance needed. Excess hydraulic fluid is bypassed internally within the pump, creating heat and excess torque, which adversely affects fuel economy.
To improve the feel of a power steering system at all speeds and to make the system more fluid economical, conventional power steering systems may use electronic variable orifice (EVO) power steering systems. In an EVO power steering system, the fixed orifice of a power steering system is removed in the pump assembly and replaced with an EVO actuator. The EVO actuator is a flow control valve that is threaded onto the outlet of the pump which regulates flow rate as a function of vehicle speed as determined by an algorithm control. The EVO system works by providing high flow rates to the steering gear at low vehicle speeds (EVO actuator fully open) and lower flow rates as vehicle speeds increase (EVO actuator begins to close). A hand wheel speed sensor is typically used to in conjunction with the EVO system to increase steering assistance when it senses that the vehicle operator is making an evasive maneuver. The excess flow that the pump produces in high or low speed situations is normally bypassed internally within the pump.
Recently, improvements have focused on alleviating the excess flow that must be bypassed internally within the pump. To accomplish this, a variable displacement pump replaces the constant displacement pump. The variable displacement pump controls pressure on the outer surface of a movable cam ring to vary the volume of fluid passing through the pump. In this way, the flow of fluid through the pump can be controlled during either low-speed or high-speed operations. Also, less fluid may be bypassed internally within the pump, decreasing excess heat and torque that affects fuel economy. However, currently available variable displacement pumps have complex designs that are expensive to manufacture.
It is thus an object of the present invention to simplify the design of the variable displacement pump by providing a variable displacement pump where the outlet port is formed integrally to the rear body of the pump.
It is another object of the present invention to simplify the design of the variable displacement pump by providing a variable displacement pump where the combination inlet port is formed integrally to the rear body of the pump.
The present invention simplifies the design of conventional variable displacement pumps by integrating the combination fluid inlet and the fluid outlet into the rear body. Although the new design adds some complexity to the rear body, the overall pump design is less costly to manufacture than previous arrangements.
Other objects and advantages of the present invention will become apparent upon considering the following detailed description and appended claims, and upon reference to the accompanying drawings.